The present invention relates to a method of preparing camptothecin and camptothecin analogs employing chemical compounds that are useful as intermediates and to processes for the preparation of the intermediates.
Camptothecin is a naturally occurring compound, found in Camptotheca acuminata. Camptothecin and camptothecin analogs have been found to have anti-leukemic and anti-tumor properties.
Camptothecin and camptothecin analogs can be synthesized using processes described in U.S. Pat. No. 4,894,456 to Wall et al. issued Jan. 16, 1990; U.S. Pat. No. 4,399,282 to Miyasaka, et al. issued Aug. 16, 1983; U.S. Pat. No. 4,399,276 to Miyasaka, et al. issued Aug. 16, 1983; U.S. Pat. No. 4,943,579 to Vishnuvajjala, et al. issued Jul. 24, 1990; European Patent Application 0 321 122 A2 filed by Smith Kline Becham Corporation, and published Jun. 21, 1989; U.S. Pat. No. 4,473,692 to Miyasaka, et al. issued Sep. 25, 1984; European Patent application No. 0 325 247 A2 filed by Kabushiki Kaisha Yakult Honsh, and published Jul. 26, 1989; European Patent application 0 556 585 A2 filed by Takeda Chemical Industries, and published Aug. 25, 1993; U.S. Pat. No. 4,981,968 to Wall, et al. issued Jan. 1, 1991; U.S. Pat. No. 5,049,668 to Wall, et al. issued Sep. 17, 1991; U.S. Pat. No. 5,162,532 to Comins, et al.; issued Nov. 10, 1992; U.S. Pat. No. 5,180,722 to Wall, et al. issued Jan. 19, 1993 and European Patent application 0 540 099 A1, filed by Glaxo Inc., and published May 5, 1993.
Previous methods used in the preparation of camptothecin and camptothecin analogs employ resolutions or chiral auxiliaries to obtain enantiomerically enriched intermediates. A problem with these methods is that a resolution necessitates discarding half of the racemic material and a chiral auxiliary requires utilizing stoichiometric amounts of a chiral subunit to stereoselectively install the chiral center.
A method which uses a process of catalytic asymmetric induction is described in U.S. patent application Ser. No. 08/237,081 and Fang et al., Journal of Organic Chemistry, 59(21), 6142-6143 (1994). One potential problem with such prior methods is that some of the chirally specific intermediates themselves may exhibit cell toxicity. Furthermore, the final step of the synthesis described in U.S. patent application Ser. No. 08/237,081 requires the use of a palladium catalyst which must subsequently be removed from the final drug substance by multiple recrystallizations. The potent cytotoxicity of camptothecin and some of its analogs requires that stringent safeguards be imposed during all the later steps of manufacturing to protect production personnel and the environment. Such safeguards increase the complexity and cost of manufacturing and handling camptothecin and its analogs.
An object of the present invention is a method for the preparation of camptothecin and its analogs wherein the chirality at the 20 position is not introduced until the penultimate manufacturing step. This would reduce the risk of accidental contamination of the environment and injury to the production worker, and hence, reduces the need for stringent safeguards, since handling and storage of highly biologically active material is minimized.
The present invention provides a method of preparing compounds of Formula (I) which comprises oxidizing compounds of Formula (II) 
wherein:
R1 and R2, which may be the same or different, are independently selected from hydrogen, lower alkyl, (C3-7)cycloalkyl, (C3-7)cycloalkyl lower alkyl, lower alkenyl, hydroxy lower alkyl, or alkoxy alkyl, or (xe2x80x94CH2NR7R8), wherein:
i) R7 and R8, which may be the same or different, are independently selected from hydrogen, lower alkyl, (C3-7)cycloalkyl, (C3-7)cycloalkyl lower alkyl, lower alkenyl, hydroxy lower alkyl, or lower alkoxy lower alkyl; or
ii) R7 represents hyrogen, lower alkyl, (C3-7)cycloalkyl, (C3-7) cycloalkyl lower alkyl, lower alkenyl, hydroxy lower alkyl, or lower alkoxy lower alkyl, and R8 represents xe2x80x94COR9,
wherein:
R9 represents hydrogen, lower alkyl, perhalo-lower alkyl, (C3-7)cycloalkyl, (C3-7)cycloalkyl lower alkyl, lower alkenyl, hydroxy lower alkyl, lower alkoxy, lower alkoxy lower alkyl; or
iii) R7 represents hydrogen or lower alkyl; and R8 represents diphenyl-methyl or xe2x80x94(CH2)tAr
wherein:
t is 0 to 5 and
Ar represents phenyl, furyl, pyridyl, N-methylpyrrolyl, imidazolyl optionally substituted with one or more substituents selected from hydroxy, methyl, halogen, and amino; or
iv) R7 and R8 taken together with the linking nitrogen form a staturated 3 to 7 atom heterocyclic group of formula (IA) 
wherein:
Y represents O, S, SO, SO2, CH2 or NR10,
wherein:
R10 represents hydrogen, lower alkyl, perhalo lower alkyl, aryl, aryl substituted with one or more substituents selected from lower alkyl, lower alkoxy, halogen, nitro, amino, lower alkyl amino, perhalo-lower alkyl, hydroxy lower alkyl, lower alkoxy lower alkyl groups or
xe2x80x94COR11,
wherein:
R11 represents hydrogen, lower alkyl, perhalo-lower alkyl, lower alkoxy, aryl, aryl substituted with one or more substituents selected from lower alkyl, perhalo-lower alkyl, hydroxy lower alkyl, lower alkoxy lower alkyl groups; or
R3 and R4 are independently selected from hydrogen, lower alkyl, (C3-7)cycloalkyl, (C3-7)cycloalkyl lower alkyl, lower alkenyl, hydroxy lower alkyl, or alkoxy alkyl; or R3 and R4 taken together form a saturated 5 to 6 atom heterocyclic group of formula (IB) 
wherein,
n represents the integer 1 or 2; or
R3 represents xe2x80x94OCONR12R13,
wherein,
R12 and R13, which may be the same or different, are independently selected from hydrogen, a substituted or unsubstituted alkyl group with 1-4 carbon atoms or a substituted or unsubstituted carbocyclic or heterocyclic group, with the proviso that when both R12 and R13 are substituted or unsubstituted alkyl groups, they may be combined together with the nitrogen atom, to which they are bonded, to form a heterocyclic ring which may be interrupted with xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94 and/or  greater than Nxe2x80x94R14 in which R14 is hydrogen, a substituted or unsubstituted alkyl group with 1-4 carbon atoms or a substituted or unsubstituted phenyl group, and
R5 represents hydrogen or alkyl, particularly methyl, and
R6 represents hydrogen or alkyl, particularly hydrogen, and
pharmaceutically acceptable salts thereof.
The present invention further provides a method of preparing compounds of Formula (I) which comprises dihydroxylating a compound of Formula (II) and subsequent oxidation to yield a compound of Formula (I).
In addition to a method of preparing compounds of Formula (I) from compounds of Formula (II), other aspects of the invention include the compounds of Formula (II) and various intermediates useful in the formation of compounds of Formula (I) and (II). Other aspects and advantages of the present invention will become apparent from a review of the detailed description below.
As used herein, the term xe2x80x9cloweralkylxe2x80x9d means, a linear or branched alkyl group with 1-8, preferably 1-4 carbon atoms, such as, methyl, ethyl, propyl, isopropyl, n-butyl, tert-butyl, hexyl and octyl. This definition also applies to a loweralkyl moiety in the loweralkoxy, loweralkylthio, and di(loweralkyl)amino groups. Thus, examples of loweralkoxy groups are methoxy, ethoxy, propoxy, sec-butoxy, and isohexoxy: examples of loweralkylthio groups are methylthio, ethylthio, tert-butylthio, and hexylthio, and examples of di(loweralkyl)amino groups are dimethylamino, diethylamino, diisopropylamino, di(n-butyl)amino, and dipentylamino.
The terms xe2x80x9chaloxe2x80x9d and xe2x80x9chalogenxe2x80x9d as used herein refer to a substitutent which may be fluoro, chloro, bromo, or iodo. The term xe2x80x9ctriflatexe2x80x9d as used herein refers to trifluoromethanesulfonate. The designation xe2x80x9cCxe2x80x9d as used herein means centigrade. The term xe2x80x9cambient temperaturexe2x80x9d as used herein means from about 20xc2x0 C. to about 30xc2x0 C.
Compounds of the present invention may have 1 or more asymmetric carbon atoms that form enantiomeric arrangements, i.e., xe2x80x9cRxe2x80x9d and xe2x80x9cSxe2x80x9d configurations. The present invention includes all enantiomeric forms and any combinations of these forms. For simplicity, where no specific configuration is depicted in the structural formulas, it is to be understood that both enantiomeric forms and mixtures thereof are represented. Unless noted otherwise, the nomenclature convention, xe2x80x9c(R)xe2x80x9d and xe2x80x9c(S)xe2x80x9d denote essentially optically pure R and S enantiomers, respectively.
Also included in the present invention are other forms of the compounds including: solvates, hydrates, various polymorphs and the like.
Acceptable salts include, but are not limited to, salts with inorganic acids and bases such as hydrochloride, sulfate, phosphate, diphosphate, hydrobromide and nitrate or salts with organic acids such as acetate, malate, maleate, fumarate, tartrate, succinate, citrate, lactate, methanesulfonate, p-toluenesulfonate, palmoate, salicylate, oxalic and stearate. For further examples of acceptable salts see, xe2x80x9cPharmaceutical Salts,xe2x80x9d J. Phamn. Sci., 66(1), 1 (1977).
One aspect of the present invention provides a method for preparing compounds of Formula (III); 
which comprises dihydroxylating a compound of Formula (II), 
using a catalytic asymmetric dihydroxylation reaction. Typically, the reaction may be carried out in the presence of an osmium catalyst (e.g., potassium osmate (VI) dihydrate, osmium(III) chloride hydrate or osmium tetroxide), a chiral tertiary amine catalyst (e.g., derivatives of the cinchona alkaloids such as hydroquinidine 1,4-phthalazinediyl diether), an oxidizing reagent (e.g., potassium ferricyanide(III), hydrogen peroxide, N-methylmorpholine N-oxide, or electricity), and a primary amide (e.g., methanesulfonamide) under basic conditions (e.g. potassium carbonate) in an aqueous mixture containing a polar protic solvent (e.g., t-butanol, i-propanol, or n-propanol). The reaction may be carried out at a temperature of between about 020  C. to about 30xc2x0 C. for about 12 to about 48 hours. Acceptable variations on these conditions are described in the literature on related catalytic asymmetric dihydroxylation reactions, e.g., K. B. Sharpless et al., J. Org. Chem. 58, 3785-3786 (1993).
Alternatively the compound of Formula II is oxidized to a compound of Formula III in an achiral dihydroxylation reaction to yield a racemic cis-diol which is then resolved enzymatically to give the enantiomerically enriched compound of Formula II. Descriptions of achiral dihyroxylations are provided by Larock, Comprehensive Organic Transformations, 493-496 (1989). The resolution reaction may be carried out in the presence of an acylating enzyme such as pancreatic lipases, Pseudomonas fluorenscens lipases, C. Cylindracea lipases, Chromobacterium viscosum lipases and Aspergillus niger lipases in the presence of an acylating agent such as vinyl acetate at a temperature of between about 0xc2x0 C. to ambient temperature for about 2 to about 48 hours. Variations on these conditions will be apparent from A. Klibanov, Asymmetric Transformations Catalyzed by Enzymes in Organic Solvents, Acc. Chem. Res. 23, 114-120 (1990).
Compounds of Formula (II) may be prepared by cyclizing a compound of Formula (IV), 
wherein X represents triflate or halo, particularly chloro-, bromo-, and iodo-.
The compounds of Formula (IV) may be cyclized by an intramolecular Heck reaction. The reaction may be carried out in the presence of a palladium catalyst (e.g., palladium(II) acetate) under basic conditions in a polar aprotic solvent (e.g. acetonitrile or N,N-dimethylformamide) or a polar protic solvent (e.g., n-propanol, i-propanol, or t-butanol). A phase transfer catalyst such as a tetraalkylammonium halide salt (eg., tetrabutylammonium chloride, tetrabutylammonium bromide, or tetrabutylammonium iodide) may be included when a polar aprotic solvent is used. Preferably, a ligand for the palladium catalyst may also be included such as a triphenylphosphine, tri-o-tolylphosphine, tri-m-tolylphosphine, or tri-p-tolylphosphine. The reaction may be carried out in an inert atmosphere, such as under nitrogen or argon gas in a suitable reaction vessel equipped with mechanical stirrer and water-cooled condenser. The reaction mixture may be heated to a temperature between about 50xc2x0 to about 110xc2x0 C. for about 1 to about 48 hours. Variations on these conditions are described in the literature on the Heck reaction, e.g., R. Grigg et al., Tetrahedron 46, 4003-4008 (1990).
The compounds of Formula (IV) may be prepared by condensing compounds of Formula (V) and Formula (VI). 
wherein,
X represents triflate or halo particularly chloro-, bromo-, and iodo-and Z represents a suitable leaving group such as chloro-, bromo-, and iodo- or OR15, wherein R15 represents triflate, mesylate, or tosylate, or particularly H.
In the case wherein Z represents hydroxy, the condensation reaction is carried out in an aprotic solvent, e.g., methylene chloride, in the presence of a trialkyl- or triarylphosphine, e.g., triphenylphosphine, and a dialkyl azodicarboxylate, e.g., diethyl azodicarboxylate, at a temperature between about 0xc2x0 C. to about 50xc2x0 C. for about 0.5 to 4 hours. Further variations on the above conditions will be apparent from the literature on the Mitsunobu reaction, e.g., O. Mitsunobu, Synthesis, 1, (1981).
When Z represents halo, triflate, mesylate, or tosylate, the condensation reaction is carried out in a polar aprotic solvent such as acetonitrile or N,N-dimethylformamide, or a polar protic solvent such as i-propanol or t-butanol, in the presence of a base, e.g., potassium t-butoxide, at a temperature between about 25xc2x0 to about 100xc2x0 C. for about 1 to 24 hours to yield compounds of Formula (IV). Variations on the above conditions are described in U.S. Pat. No. 5,254,690 to Comins et al. issued Oct. 19, 1993 and incorporated herein by reference.
Compounds of Formula (VI) may be prepared from compounds of Formula (VII), 
wherein,
R16 represents alkyl, particuarly methyl.
The dealkylation reaction may be carried out in a polar aprotic solvent, e.g. acetonitrile, in the presence of a suitable dealkylating reagent, e.g., a trialkylsilyl iodide, at a temperature between about 0xc2x0 C. and 100xc2x0 C. for about 1-12 hours. The trialkylsilyl iodide may be generated in situ by combining a trialkylsilyl halide, e.g., trimethylsilyl chloride, and an alkali metal iodide, e.g., sodium iodide.
Alternatively the dealkylation reaction may be carried out in a polar, protic solvent, e.g., water or ethanol, in the presence of a strong acid, e.g., hydrochloric acid at a temperature between about 0xc2x0 C. and 100xc2x0 C. for about 1 to 24 hours to yield the compound of Formula (VI).
The starting materials, the compounds of Formula (V) and Formula (VII), are described in U.S. patent application Ser. No. 08/237,081, Fang et al., Journal of Organic Chemistry, 59(21), 6142-6153 (1994), PCT/US95/05425, and PCT/US95/05427.
The compounds of Formula (III) may be oxidized to yield a compound of Formula (I). 
The oxidation reaction may be carried out in a suitable solvent, e.g., methylene chloride, in the presence of an oxidizing agent, e.g., dimethylsulfoxide, an activating reagent, e.g., oxalyl chloride, and a base, e.g., triethylamine, at a temperature between about xe2x88x9278xc2x0 C. and xe2x88x9220xc2x0 C. for about 0.1 to about 1 hours to yield a compound of Formula (I). Further variations on these conditions will be apparent from the literature on activated sulfur-based oxidants, e.g., Mancuso and Swern, Synthesis, 165-185 (1981) and March, J., Advanced Organic Chemistry, 3rd edition, John Wiley and Sons, New York (1985), pp. 1057-1060, 1081-1082.
Thus, progressing compounds of Formula (V) and (VI) to compounds of Formula (I) through the intermediate compounds of Formula (IV), (II), and (III) is schematically represented by the following scheme: 
A further aspect of the invention are the novel compounds of Formula (II), (II), (IV), and (VI).
The compounds of Formula (It), (III), (IV), (V), (VI), and (VII) are useful as intermediates in the preparation of camptothecin and camptothecin analogs, e.g. compounds of Formula (I), and those described in European Patent application 0 540 099 A1, filed by Glaxo Inc., and published May 5, 1993 and incorporated herein by reference.
A typical preparation of a camptothecin derivative of Formula (I) using intermediate compounds of Formula (II), (III), (IV), (V), (VI), and (VII) is exemplified herein.